1. Field of the Invention
This invention relates to a chemically modified mica composite having a layered structure, and more particularly to mica composites which exhibit ion exchange properties, and are particularly designed to be excellent ion exchangers for cesium.
2. Description of the Prior Art
Ever since nuclear weapons were first produced at the end of World War II, large amounts of nuclear waste have been generated and stored at various facilities. The nuclear waste, which consists largely of the byproducts of uranium and plutonium production and purification, was disposed of in ways which were deemed suitable at the time, but which in retrospect are now inadequate. Much of the nuclear waste is now stored in tanks as a highly alkaline mixture of salts and liquids which, if not recovered and properly remediated, will potentially create severe environmental problems.
Most of the stored aqueous nuclear waste is alkaline (pH 14), and contains high concentrations of sodium nitrate. The tanks contain various complexing agents, fission products, transuranic elements and other materials. Much of the stored nuclear waste is in the form of sludge created when alkali was added to the waste to prevent tank corrosion. Some of the radioactive material has been incorporated into salt cakes which is the evaporative product of the alkaline aqueous material. It is desired to remove the radioactive elements from the waste in order to allow for subsequent safe disposal of the non-radioactive materials. The removal of two of the metallic radionuclides, cesium and strontium, is particularly important because their half-lives are long enough to represent a hazard for an extended period of time.
Use of layered inorganic materials to remove cesium from aqueous solution is known to those in the art. J. S. Wahlberg and M. J Fishman Adsorption of Cesium on Clay Minerals, Geological Survey Bulletin 1140-A (1962) describe adsorption of cesium on several clays, though no micas.
Dyer and Gallardo in Recent Developments in Ion Exchange pp. 75-84 (1990) reported the use of bentonite clays as ion exchange materials and measured cesium uptake capacities. However, they did not show that any significant selectivity exists for cesium over sodium. They also reported pillaring of these clays with zirconium.
Preparation of sodium form micas from the naturally occurring potassium micas is known to be extremely difficult. Mortland (Soil Sci. Soc. Am. Proc. (1958) 22 503-508) leached a mica sample with large volumes of 0.01N sodium chloride and was able to extract most of the potassium from it, but the procedure is clearly not scaleable for industrial use. Robert and Pedro (Int. Clay Conf. Proc. (Tokyo, Japan) (1969) 1, 455-473) used sodium cobaltonitrite to extract potassium from biotite samples. Six successive treatments were required, and this number of treatments, and the expense of the reagent makes this route unattractive for large scale use. Scott and Amonette ("Role of Iron in Mica Weathering" Iron in Soils and Clay Minerals, J. Stucki ed. (1988) 584-623) have similarly used sodium tetraphenylborate to precipitate the potassium as it is removed from the mica, thereby improving the efficiency of the removal operation. However, this procedure results in a mixture of two solids, the potassium tetraphenylborate and the mica, and these solids are then difficult to separate. The expense of the reagent also makes this route unattractive for large scale use.
Komarneni and Roy (Science 239, 1286 (1988)) disclosed that micas prepared by the Scott/Amonette method are effective for the removal of cesium from aqueous solutions containing sodium. They did not disclose any more effective methods for the preparation of this material, nor did they disclose that partial or complete ion exchange of the sodium ions by other larger cations increases the selectivity of the material.
Increasing the interlayer spacing of micas by insertion of silica pillars is described in J. W. Johnson and J. F. Brody (U.S. Pat. No. 5,330,734 (1994)), and with alumina pillars in European Patent Application 0240359 B1. Preparation of zirconia-pillared fluormicas is disclosed in Johnson et al. in Chem. Mater. 5, 36-42 (1993). Sekimoto, Kondo and Saiki (JP 06287014 (1993)) disclose the intercalation of quaternary ammonium salts into clays or micas. However none of these references disclose the advantages that these modifications to mica have on the performance of the material as an ion exchanger for cesium.